1. Field of the Invention
The present invention relates to the retrieval of submerged pipelines, and more particularly to an improved recovery head for recovering wet buckled pipe.
2. Description of the Related Art
Marine pipelines are typically laid on the sea floor from floating vessels called lay barges. To install a submerged pipeline, joints of pipe are welded together on the lay barge. The end of the first joint of pipe is sealed to prevent water from filling the pipeline as the pipeline is laid. As the joints of pipe are welded together into a continuous length of pipeline, the lay barge is moved forward in the water and the welded pipeline slides on pipeline rollers on the lay barge, then over a stinger which is attached to the stern of the lay barge and into the water. As the pipeline leaves the lay barge and the stinger, it bends under its own weight and assumes a generally S-shaped configuration between the stern of the lay barge and the sea floor. The stinger is an extended support which limits the bending of the pipeline between the lay barge and the sea floor, thus reducing the potential of the pipeline buckling as it is being laid. Pipeline tensioners on the deck of the lay barge grip the pipeline and further reduce the bending of the pipeline as it is being laid.
During pipe lay operations, loss of tension in the pipeline due to adverse weather or to failure of the tensioners can cause the pipeline to bend excessively. Excessive bending of the pipeline may cause the pipeline to buckle. If the buckle results in water entering the pipeline it is referred to as a "wet buckle." The submerged pipeline will fill with water, with the pipeline breaking at a location, generally near the stinger, and fall to the sea floor. A pipeline can also be damaged after the pipeline has been laid, for example, by ships dragging their anchors over the pipeline.
Normally, to repair a submerged pipeline which has buckled and broken or has been damaged, the broken or damaged end of the pipeline needs to be lifted to the water surface so that the pipeline can be repaired. The depth of the water typically determines whether or not the pipeline end will be recovered using divers or diverless vehicles. Conventional pipeline recovery techniques using divers are restricted to maximum depths of about 300 meters. In water depths which exceed 300 meters, the typical recovery method utilizes a remotely operated vehicle (ROV) which is controlled by operators from a vessel on the water surface. Regardless of the water depth, the typical recovery method involves cutting the pipeline at the sea floor and then inserting a lift or recovery head into the open end of the pipeline section that is to be raised to the water surface. A retrieval cable is then attached to the lift head, which is then drawn in by a winch on the lay barge to raise the pipeline end to the water surface. In deep water, the ROV manipulates cutting equipment to sever the damaged pipeline and then inserts the lift head into the open end of the pipeline section that is to be raised to the water surface.
It is desirable that the recovery head be both stable and maneuverable to enable divers or the ROV to more easily manipulate the recovery head into the pipe end. In addition, the manner in which the retrieval cable is attached to the recovery head must be relatively simple to allow an ROV to easily attach the cable to the recovery head. This is especially important in the case of a large or heavy retrieval cable where the ROV is unable to easily manipulate the cable. Also, the attachment must be relatively secure to guarantee that the pipe end can be raised to the surface without incident.
When the recovery cable is reeled into the lay barge, the pipe end is retrieved onto the stinger and then onto the pipe ramp and into the grasp of the pipeline tensioners on the lay barge. It is generally desirable for the recovery head to be not greater in diameter than the pipeline so that the recovery head does not obstruct or interfere with any steps involved in the actual retrieval and securing of the pipeline on the lay barge.
The recovery heads currently in use generally do not include any type of stabilization mechanism to allow an ROV to more easily manipulate the recovery head into the pipe end. In addition, it is desirable that any stabilization mechanism added to a recovery head not interfere with the actual retrieval and securing of the pipeline on the lay barge. Also, the recovery heads used in the prior art generally include a hole in the rear end of the head to which a hook may be attached to retrieve the head to the water surface. It has been found to be difficult for an ROV to manipulate the hook into the hole provided in the recovery head. Hence, a simplified mechanism for attaching a retrieval cable to a recovery head is needed.
Therefore, an improved recovery head is needed which is stable and maneuverable such that the head can be easily inserted into the pipeline. The retrieval cable must be easily attached to the recovery head and must also be secure to enable the recovery head and pipeline to be easily pulled to the surface. In addition, the recovery head must be such that the retrieved end can be directly pulled onto the stinger and into the grasp of the pipeline tensioners, with the recovery head small enough to pass through the tensioners on the lay barge.